USB: cdc-acm: Remove obsolete predefined speeds array

Modern speed handling has been introduced in 2009 by commit
9b80fee149a875a6292b2556ab2c64dc7ab7d6f5 (cdc_acm: Fix to use modern
speed interfaces) and the acm_tty_speed array has been unused since.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>